How Many Watts Does a House Need in an Outage?
There is no single answer, because a house does not have one power requirement — it has three, depending on how much of normal life you want to keep.
Alex Rivers
Home Improvement Editor
Last Updated
September 2, 2026
In This Guide
People ask how many watts a house needs as though the house were one appliance. It is not. The honest answer is that keeping the food cold and the heat on costs a fraction of what keeping the air conditioning running costs, and the gap between those two numbers is the whole decision.
1. The Three Tiers of Outage Power
Rather than one number, think in tiers. Each buys a distinct standard of living during an outage, and each maps onto a different class of generator.
| Tier | Watts Needed | What You Get | Typical Machine |
|---|---|---|---|
| Survival | 2,000-3,500 | Fridge, freezer, some lights, phone charging | Small inverter or portable |
| Essentials | 3,500-7,500 | Above plus furnace blower, sump pump, internet | Mid-size portable |
| Comfort | 7,500-12,000 | Above plus well pump, window AC, microwave | Large portable or small standby |
| Whole house | 18,000-22,000+ | Above plus central AC, electric range, dryer | Standby unit |
Most households are genuinely served by the Essentials tier. Keeping the refrigerator and freezer cold, the heating system circulating, the sump pump protecting the basement and the lights and internet on covers the great majority of what an outage actually threatens. That is a 5,000 to 7,500 watt machine, not a 22,000 watt one.
Where the Number Explodes
Anything that makes heat or moves a lot of air.
Central air conditioning, electric water heaters, electric ranges and electric dryers are the loads that triple a household total. A three-ton central air conditioner alone runs at 3,000-3,800 watts and surges to 9,000-12,000. Deciding whether you need those during an outage is the single biggest factor in what you will spend.
2. Why Two Numbers, Not One
Every appliance has two wattage figures and confusing them is the most common sizing mistake.
Running watts, sometimes called rated watts, is the continuous draw once an appliance is operating. Starting watts, or surge watts, is the much larger momentary draw as a motor overcomes inertia and spins up. The surge lasts a second or two, but if the generator cannot supply it, the appliance will not start and the generator may trip.
The gap between the two
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator or freezer | 150-800 | 1,200-2,200 |
| Gas furnace blower | 600-900 | 1,500-2,400 |
| Sump pump, 1/2 HP | 800-1,050 | 2,000-2,600 |
| Well pump, 1 HP | 1,000-1,200 | 3,000-4,000 |
| Central AC, 3 ton | 3,000-3,800 | 9,000-12,000 |
| Lights, TV, router, laptops | 300-600 | Same |
Notice the pattern: motors surge, resistive loads do not. Anything with a compressor or a pump has a large gap between the two columns. Anything that simply makes heat or light — a kettle, an incandescent bulb, an electric heater — draws the same at start-up as it does in operation.
The Rule for Combining Them
You do not add every appliance’s starting watts together, because they do not all start at the same instant. The correct method is to total the running watts of everything you want on, then add only the single largest start-up surge on the list. That represents the worst realistic moment: everything is already running when the biggest motor kicks in.
3. Three Worked Examples
A Small Home, Gas Heat, No Basement
Refrigerator at 700, lights and electronics at 500, gas furnace blower at 900. Running total is 2,100 watts. The largest surge belongs to the furnace blower, adding roughly 1,500. Peak demand is about 3,600, and with twenty percent headroom you are looking at around 4,300 watts. A 5,000 watt portable is comfortable.
A Family Home With a Sump Pump
Refrigerator 700, freezer 500, lights and electronics 600, gas furnace blower 900, sump pump 1,050. Running total is 3,750. The largest surge is the sump pump at roughly 1,550 above its running draw. Peak is about 5,300, and with headroom around 6,400 watts. A 7,500 watt portable is the right machine, and this is the most common residential scenario.
The Same Home, Plus Central Air Conditioning
Add a three-ton central air conditioner at 3,800 running. The running total jumps to 7,550. The largest surge is now the air conditioner at roughly 8,200 above its running draw. Peak demand is about 15,750, and with headroom close to 19,000 watts. You have left portable territory entirely and are shopping for a standby unit.
What Those Three Examples Show
Air conditioning roughly triples the requirement.
The first two scenarios differ by a single sump pump and 2,000 watts. The third differs by one appliance and nearly 13,000 watts. If you are trying to decide between a large portable and a standby installation, the question is almost always whether central air conditioning is on the list.
4. How to Find Your Own Appliance Wattages
Published tables are useful for planning, but your appliances are the ones that matter, and they can differ substantially from typical figures.
Look for the data plate
Most appliances carry a label on the back, base or inside the door listing watts, or listing volts and amps. It is the manufacturer’s own figure for that specific unit.
Multiply volts by amps if watts are not listed
A label showing 120V and 6A means roughly 720 watts. This covers most appliances that give amperage rather than wattage.
Check the motor rating for pumps and compressors
Horsepower ratings on pumps translate roughly: 1/2 HP is around 1,000 running watts, 1 HP around 1,200, with surges two to three times that.
Look up your HVAC equipment specifically
Furnace blowers and air conditioners vary enormously. The model number and a manufacturer specification sheet give you a real figure rather than a range.
Use a plug-in meter for anything uncertain
An inexpensive energy monitor plugged in between the outlet and the appliance shows real running draw. It will not usually capture the brief surge, but it settles the running figure.
Add it up with the running-plus-largest-surge rule
Total the running watts, add the single biggest surge, then add about twenty percent of headroom.
The twenty percent headroom is not padding for its own sake. Generators are not designed to run at their maximum rating continuously — doing so shortens engine life, increases fuel consumption disproportionately and leaves nothing in reserve when an unexpected load starts. Sizing to about eighty percent of rated output is the standard practice.
5. What to Deliberately Leave Off the List
The cheapest way to lower the number is to decide that some things simply do not run during an outage. These are the usual candidates, and skipping them frequently drops a household from standby territory back into portable territory.
- Electric dryer at 5,000-5,500 watts — clothes can wait, or dry on a rack.
- Electric water heater at 4,000-4,500 watts — most tanks hold usable hot water for a day or more.
- Electric range at 1,500-2,500 watts per element — a microwave or a camping stove costs far less power.
- Central air conditioning at 3,000-3,800 running and up to 12,000 surge — a single window unit covers one room for a fraction.
- Second refrigerators and beverage coolers — consolidate into one during the outage.
One important caveat: managing loads manually only works when someone is home to manage them. A standby generator running a whole house does not require anyone to think about what is switched on. If the house may be empty during outages, that convenience is part of what you are buying.
The other approach worth considering is load shedding, where a transfer switch or smart management module automatically sheds lower-priority circuits when demand approaches the generator’s limit. That allows a smaller unit to serve a larger house by never letting everything run at once, and it can make a mid-size standby generator viable where a full-capacity one would be needed otherwise.
6. What Each Wattage Band Buys You
Wattage figures only mean something once mapped onto machines you can actually buy. These are the standard bands and what each realistically covers.
| Generator Size | Type | Realistically Runs |
|---|---|---|
| 2,000-2,200W | Small inverter | Fridge and lights, one at a time with care |
| 3,500-4,000W | Portable | Fridge, lights, furnace blower |
| 5,000W | Portable | Above plus a sump pump comfortably |
| 7,500W | Portable | Full essentials with headroom; the common choice |
| 9,500-12,000W | Large portable | Above plus a well pump or window AC |
| 14,000-18,000W | Standby | Most of a house, possibly with load shedding |
| 22,000W+ | Standby | Whole house including central air conditioning |
Why 7,500 Watts Is the Common Recommendation
It is not arbitrary. The standard essentials list — refrigerator, freezer, gas furnace blower, sump pump, lights and internet — works out to roughly 3,750 running watts, and the largest surge on that list adds about 1,550. Peak demand lands near 5,300, and adding the twenty percent headroom that keeps a generator off its rated limit gives around 6,400. The next standard size up is 7,500, which is why that figure appears so consistently in recommendations.
When to Buy the Next Size Up
Two situations justify going larger than the arithmetic suggests. A well pump surging to 4,000 watts substantially raises the peak and narrows the margin on a 7,500 watt machine considerably. And if there is any prospect of adding a window air conditioner, a second freezer or a home office later, the incremental cost of the next size up now is far smaller than replacing the whole generator in two years.
When Not to Oversize
Bigger is not free. A larger generator costs more to buy, burns more fuel at any given load because the engine itself is larger, is heavier to move, and is louder. A generator running at a small fraction of its rated capacity is also running inefficiently. Size for your genuine list plus headroom rather than buying the largest machine the budget allows.
7. Common Mistakes to Avoid
Adding Every Appliance’s Starting Watts Together
Motors do not all start at the same instant, so summing every surge produces a wildly oversized and expensive number. Total the running watts of everything you want on, then add only the single largest start-up surge. That is the worst realistic moment and the figure to size against.
Sizing to the Generator’s Maximum Rating
A generator running continuously at its full rated output burns fuel disproportionately, runs hot and leaves nothing in reserve when an unexpected motor starts. Size for about eighty percent of rated output — add roughly twenty percent headroom to your peak figure and buy the next size up.
Forgetting the Loads That Are Hardwired
Furnaces, sump pumps and well pumps are usually wired directly into the panel and cannot be reached with an extension cord. If they are on your wattage list, you also need a transfer switch or interlock kit — the wattage calculation alone does not get power to them.
Using Generic Figures for HVAC Equipment
Furnace blowers and air conditioners vary far more between models than kitchen appliances do, and they are usually the largest loads on the list. Find the model number and the manufacturer’s specification sheet rather than relying on a typical range for the one appliance where being wrong is most expensive.
Assuming You Need Whole-House Capacity
Most households are genuinely well served at the essentials tier — fridge, freezer, furnace blower, sump pump, lights and internet on a 7,500 watt machine. Central air conditioning is what pushes a house from a portable to a standby unit, so decide whether you actually need it before shopping for capacity you may never use.
8. Frequently Asked Questions
How many watts does it take to power a house?
It depends on what you want running. Essentials — refrigerator, freezer, furnace blower, sump pump, lights and internet — typically need 3,500 to 7,500 watts. Adding a well pump and window air conditioning pushes it to 7,500-12,000. A whole house including central air conditioning and electric appliances needs 18,000-22,000 or more.
Will a 7500 watt generator run a house?
It will run the essentials of most homes comfortably: refrigerator, freezer, gas furnace blower, sump pump, lights, internet and small appliances, with headroom for the largest start-up surge. What it will not usually do is start a three-ton central air conditioner, which surges to between 9,000 and 12,000 watts.
How do I calculate how many watts my house needs?
List the appliances you want running, total their running watts, then add only the single largest starting surge on that list — motors do not all start at once. Add about twenty percent headroom and buy the next standard generator size up from that figure.
What uses the most watts in a house during an outage?
Anything that makes heat or moves a lot of air. A three-ton central air conditioner runs at 3,000-3,800 watts and surges to 9,000-12,000. Electric dryers draw 5,000-5,500 and electric water heaters 4,000-4,500. Leaving these off a backup list is usually what keeps a household in portable generator territory.
How many watts to run a refrigerator and freezer?
A refrigerator or freezer typically runs at 150-800 watts but surges to 1,200-2,200 when the compressor starts. Two units together might run at 1,200 watts continuously, but you must size for the largest surge on top of that, so allow roughly 3,000 watts for the pair plus lighting.
Do I need to size for starting watts or running watts?
Both, combined correctly. Size for the total running watts of everything you want on, plus the single largest starting surge among them. Running watts alone will leave the generator stalling when a pump or compressor kicks in; adding every surge together produces a needlessly oversized machine.
Is a 10000 watt generator enough for a house?
For most homes without central air conditioning, comfortably yes — it covers the essentials plus a well pump, a window air conditioner and a microwave with headroom. It is generally not enough for a three-ton central air conditioner, which surges to between 9,000 and 12,000 watts on its own.
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The Bottom Line
A house does not have one wattage requirement. It has a survival number around 2,000 to 3,500 watts, an essentials number around 3,500 to 7,500, a comfort number around 7,500 to 12,000, and a whole-house number of 18,000 to 22,000 or more. Which one applies to you is a decision about how much of normal life you want to keep, not a fact about your house.
Most families are genuinely well served at the essentials tier. Refrigerator, freezer, gas furnace blower, sump pump, lights and internet is a comfortable outage, and it is a 7,500 watt portable rather than a standby installation. What pushes a household into standby territory is almost always central air conditioning, which alone can add 12,000 watts of surge demand.
Whatever tier you choose, do the arithmetic the right way: total the running watts, add only the single largest start-up surge, then add about twenty percent of headroom so the generator is not pinned at its rating. Get that calculation wrong in the common way — by summing every appliance’s surge — and you will buy far more generator than you need.
And remember that the tier you choose is a decision you can revisit. Load shedding, a smaller generator run thoughtfully, or simply accepting that the dryer waits until the power returns are all legitimate ways to bring a large house within reach of a modest machine.
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About the Author
Alex Rivers, Home Improvement Editor
Alex has spent over a decade working on residential and light commercial property maintenance, and now tests every tool, coating and machine that appears in these guides personally.